iPhone - UITableViewCell white after selected

How is it possible that my UITableViewCell (Static cells) after i selected it become white again? When i tap on it , it becomes grey and stays grey! It should be grey for a short time if i tap on it.

Thanks

EDIT

Normally when you tap on a UITableViewCell - the background color of the cell turns grey for a short time. But my problem is in my application, when i tap on the cell, the background color of the cell stays grey. And I don't know how to solve this problem.